The characterization of surfaces and interfaces and of interactions taking place at those surfaces and interfaces is important for answering many questions arising in chemical, biotechnical and medical processes. In particular, for evaluating aqueous suspensions containing solids, emulsifying agents, fibers and other particles. The characterization of surfaces and interfaces and the interactions taking place at those places is of great importance in paper manufacturing. Electric effects at solid-liquid phase boundaries, the electric double layers and the related zeta-potential of the solid are characteristic for the respective material and its actual environment. The electric potential of the solid surface affects the absorption and adhesion of materials from the corresponding environment. The magnitude and the mathematical sign of the surface charge can be determined by measuring the so-called zeta-potential which describes the galvanic voltage at the diffuse electrochemical double-layer at the phase boundary between the surface of a solid and a fluid.
The zeta-potential of fibers in a fiber suspension is an important parameter in the paper industry for guaranteeing an optimal process flow. The same applies for textile fibers in the textile industry and for many types of particles in industrial processes. Several methods exist for determining this potential, for example:                Measurement of the drift velocity of the particles in the suspension in an electric field, from which the zeta-potential is computed,        Measurement of the streaming potential of fibers or particles in the suspension, computation of the zeta-potential from the measured streaming potential by using an empirical formula.        
EP 0 462 703 B1 describes a device for measuring an electric property, of a fiber dispersion. This device for measuring a pressure-dependent characteristic of a dispersion of solid material in a fluid consists of a means for transporting at least part of the fluid from a first chamber through a sieve into a second chamber for forming on the sieve a cushion of solid material and means for measuring the characteristic. This device has a pressure control device with at least one differential pressure controller, wherein the differential pressure controller is arranged such that a pressure signal commensurate with a predetermined pressure value is to be defined, and has additional means for withdrawing air from a second chamber with a defined velocity.
DE 43 45 152 A1 describes a zeta-potential measurement cell. The zeta-potential measurement cell for determining the zeta-potential on exterior and/or swelling surfaces of materials, which are stable under streaming conditions, includes a body which is provided with at least two intersecting through bores enclosing an angle of 90°. In one of the through bores, a rotatable and displaceable die, which is sealed against the body, is inserted from each opening of the through bore until the spacing is equal to the measurement gap. The end faces of the dies in the body are flat and mutually parallel, with a measurement gap located in between. The other through bore is formed as an entrance and exit channel for an electrolytic fluid, with the entrance and exit channel each having a respective electrode. The surface of the entrance and exit channel and of the dies in the body prevent direct electric connection between the two electrodes.
WO 97/36173 A1 describes a device for determining the charge density of dissolved, colloidal dissolved or undissolved, organic or inorganic materials in a sample fluid by titration, with a measurement container having at least two electrodes for receiving the sample fluid, with a piston which can be moved in the sample fluid with a motor about an operating position arranged in the measurement container, characterized in that stripper means for mechanically cleaning the piston and the vessel are provided.
A device for electro-kinetic analysis with a minimal fluid volume is described in DE 202 09 563 U1. The device is characterized in that an oscillating fluid stream is generated which can operate with small sample and fluid volumes, wherein in fibrous, powdered or granulated sample material the ratio of fluid volume to packed volume of the solid sample must not be greater than 10:1, and for planar samples the ratio of fluid volume to solid surface must not be greater than 0.5 cm3/cm2.
Other devices for measuring the streaming potential of fiber- and/or particle-containing aqueous suspensions operating according to various functional principles are known. In these devices, a fiber stopper or particle stopper is produced, for example, in a measurement cell which is open at the bottom and includes a suction tube, and which is closed on the top side with a sieve. This is attained by applying on a top sieve surface of the measurement cell a defined vacuum, thereby suctioning the suspension from, for example, a beaker and filling the measurement cell. This vacuum is generated with an external vacuum pump which is connected to a vessel connected with the measurement cell, wherein the vessel is located above the measurement cell and has a volume sufficient to receive the entire filtered matter/electrolyte of the suspension.
The suspension is suctioned via the preferably vertical suction tube located at the bottom side of the measurement cell, thereby forming the plug required for the measurement, because the sieve passes only water or the electrolyte, but practically no fibers or particles. The formed plug is simultaneously compressed by the reduced pressure in a manner required for an accurate measurement. The vacuum, which is permanently applied on the side of the sieve, ensures that the plug remains at its position. The filtered matter suctioned through the plug by the reduced pressure is collected in the vacuum vessel.
After the plug is formed, and even while the plug is formed, a periodical change in the reduced pressure produces a periodically change in the flow of the water of the suspension from the beaker through the plug. This flow produces a periodically change in the voltage caused by a deformation of charge clouds extending around the fibers or particles. The frequency of this periodic change is in the range of about 0.5 Hz to 10 Hz. The so-called streaming potential is measured with two electrodes located at the two ends of the measurement cell. The electrodes are made, for example, of stainless steel, platinum, silver or gold.
The zeta-potential is computed from the periodically changing streaming potential and the likewise periodically changing pressure difference relative to the ambient pressure, as measured with a pressure sensor, as well as from other variables. Advantageously, the periodic voltage change makes it possible to filter out DC offset voltages which can be generated, for example, by contamination and deposits on the electrodes. The periodically changing vacuum which produces the periodic flow through the plug, is produced with a powerful vacuum pump which is connected via two pressure reducing valves and two downstream valves and which is permanently active during the entire measurement. By alternatingly switching the valves, the vacuum in the vessel changes according to the reduced pressure defined by two pressure reducers. The vessel simultaneously receives the water or electrolyte which is suctioned from the beaker together with the suspension and flows through the plug and is filtered by the plug.
This process can be continued until the beaker with the suspension is empty. The plug is then removed from the vacuum vessel by applying ambient pressure to the upper portion of the measurement cell and as a result of the water flowing out due to gravity. The plug falls back into the beaker via the suction tube. Application of this functional principle is described, for example, in DE 102 00 654 A1.
Because the reduced pressure must periodically alternate in the vacuum vessel to produce the periodic fluid stream required for measuring the streaming potential, the relatively large volume of the vacuum vessel must be periodically switched from one value for the reduced pressure to the other value. In order to work at the required switching rate, a powerful and large vacuum pump is required, which is disadvantageous in particular, if suppliers of chemicals use the measurement device, for example, for different customers. The vacuum pump weighs approximately the same as the actual measurement device. The measurement device is therefore difficult to handle which complicates transport of the entire equipment.
In the aforedescribed state-of-the-art example, the temporal curve of the reduced pressure at the measurements cell and hence the streaming potential is periodically increasing and decreasing with a saw-tooth pattern, whereby the ratio of the increasing or decreasing section of the pressure curve and streaming potential curve in relation to the relatively constant section is very unfavorable due to the limited capacity of the vacuum pump and the required switching frequency of, e.g., 0.5 Hz. This means that there effectively exists no constant state, which complicates an exact computation of the zeta-potential, because the signals cannot be processed, for example, by filtering, due to their time dependence. However, exact measurement results can still be obtained by computing the zeta-potential including its mathematical sign from, for example, the streaming potential curve and the pressure curve through cross correlation.
In another method disclosed in WO 2004/015410 A1, the plug is produced in a container which is closed at one side with a sieve and is also closed with a sieve on the other side after the formation of the plug, either manually or by using a device. Thereafter, water or the electrolyte are pressed with periodically changing direction through the plug using one or two opposing motor-driven piston pumps, producing a periodic, in particular sinusoidal streaming potential, which is measured with electrodes. The zeta-potential can be computed from the streaming potential in conjunction with the measured difference pressure.